Efficiency is corn weed control goal

Paul L. Hollis Farm Press Editorial Staff | Apr 13, 2005

The most efficient and effective weed control program for corn is one that is targeted at the weed spectrum, with applications that are timely and made at the correct rate, and with sprays that are made with equipment that has been maintained.

“It's not always the cheapest program,” says University of Georgia Extension agronomist Dewey Lee. “Sometimes, you have to choose the most expensive way to be the most effective and efficient when it comes to maintaining yield.”

These programs, adds Lee, can be similar both for conventional and conservation-tillage production systems. “If you're in a conventional production system, you'll take care of a lot of weed pressure by harrowing, disking and preparing the ground. If you're in a burndown operation, your options are glyphosate and paraquat. And with our weed spectrum, including cutleaf evening primrose and others that tend to be tolerant of burndown materials, you'll need something like 2,4-D or atrazine,” he says.

When it was first introduced, says Lee, 2,4-D was seen as both a broadleaf and a grass control material. “You actually can reduce the vigor of grasses and get some crabgrass control with something like 2,4-D, particularly with good moisture. A burndown option of this nature would need to be applied one to three weeks ahead of planting corn, depending on the herbicide,” says Lee.

The first rule in planting corn is to start with a clean seedbed, he says. If the seedbed isn't clean, you'll be behind before you even get started, he adds.

“Atrazine is the cheapest material to use here, at about $4 to $7 per acre. It probably should be the foundation of your weed control program. It's inexpensive, and it has a broad spectrum of control. We use up to 2.5 pounds, and sometimes we use less than that,” says Lee.

Atrazine is weak on grasses like Texas panicum, which is the major grass problem for Georgia growers, he says. “Prowl or Pendimax — the pendimethalin materials — can be a very effective and inexpensive way to control Texas panicum. Is it the best system? Data shows it may or may not be, depending on your weed population. It's important to look at your soil type and the population of Texas panicum in your field. It costs $7 to $8 per acre, which is certainly less expensive than Accent or Option. Today, with the cost of Roundup dropping because of generic glyphosate formulations, it is becoming a more viable post-control material, especially when compared with some of our sulfonyl urea herbicides.”

The Liberty Link system also is an option for corn producers, says Lee. “I've seen situations in Georgia where Liberty Link is a very effective and efficient program. There are other options, such as post-directed application of Evict and paraquat. This is an inexpensive approach to controlling broad-spectrum weed populations.”

Whether you're in a conventional or Roundup Ready system, management decisions are required before making the first weed control application, says Lee.

“The good news about the Roundup Ready corn weed control system is that glyphosate is getting cheaper because of the generics. The bad news is that the technology does cost, and it'll probably average $20 per bag over a comparative conventional hybrid. The technology will cost you $6 to $8 on a per-acre basis.”

In field trials, Accent was coupled with atrazine at 1.5 quarts per acre with oil, costing about $26 on a per-acre basis, says Lee. “While we could still see some panicum in this trial, what you don't see is that later in the season, the panicum doesn't do anything because you've stopped it.

“You can get similar control with one application of Roundup. But with the technology fee, the application, and the cost of the material, it's about a $16 to $17 per-acre treatment when compared with the ‘Cadillac’ treatment of Accent plus atrazine plus oil. However, it may take two applications to get good control, which increases the cost and makes it similar to atrazine plus the sulfonyl urea herbicides.”

Roundup Ready corn hybrids have improved considerably, says Lee. “Several years ago, when the technology was first introduced, I felt that pound-for-pound, we'd be better off if we waited until the technology was placed in hybrids more adapted to our area. Today, those hybrids are here, and they offer exceptional use of your money, and a good weed control program.”

Looking at grain tests across the Coastal Plain Region, the Roundup hybrids averaged about 187 bushels per acre, he says. Some short-season varieties yielded in the 190-bushel range and some mid-season varieties were in the 200-bushel range.

It would be an understatement, says Lee, to say that there's generally an aversion among corn growers to making two weed control applications because of the cost.

“If you have this aversion, and you're going to use one application, you certainly need to use atrazine. Timing will be important — about three to four weeks after planting. You're talking about a 21- to 28-day period of susceptibility, and the effective and efficient use of that herbicide is going to peak during that period of time.”

Two applications of a quart-per-acre of a generic glyphosate is relatively inexpensive, he says, at about $25 per acre. “It ranks right in there with Accent, atrazine and oil combinations and other conventional programs, so they're equal.”

Looking at actual control, four irrigated locations were tested in 2003 and 2004, says Lee. The hybrid DeKalb 6760 was used in all tests, he says.

In plots with Texas panicum infestations, Roundup WeatherMax was used at 22 ounces, and there was a considerable yield difference between one and two applications, he says. “When we split the two applications, we made an additional 16 bushels of corn. At $2 corn, that's an additional $32, and it pays to make that second application.

“Is that second application necessary? In this case, it would have been necessary. This is because of the density of Texas panicum in the plot. Looking at rates of glyphosate and the new technology, there isn't a lot of change in the normal rates. It's important to improve your timing and use drop nozzles to give you better control underneath the canopy.”

Timing is everything when using Prowl, says Lee. Timing not only is everything for crop safety but also for the control of Texas panicum, he adds.

“If you use Prowl as an early post treatment, I'd suggest that you apply it in between the spiking stage and when that cotyledon leaf begins to split and expose itself. Prowl won't give control alone of emerged grasses. For that reason, as you move later into the season, Prowl becomes less of an effective and efficient chemical simply because you have grasses emerging with the corn crop.”

A timely application of Pendimax and atrazine with a crop oil gives good control, he says, but how long does it last? “Where we're effective, we'll probably get, at best, between 80 and 90 percent control. With Roundup and Accent, we're talking about 95 to 98 percent control. As we get into the growing season, we see that the residual effects of those products decline. And, after about 40 days, we see a rapid decline.

“When we've planted in March and applied a little later, we've seen considerable yield advantages with pendimethalin and atrazine versus atrazine alone or versus some of the other chemistries. It's an efficient and effective chemistry, but it will not control Texas panicum that comes up at about the same time as the corn is emerging.”

Accent and Option also were evaluated across four locations, says Lee. They were applied 21 to 24 days after planting, with an early April planting date. An average of 200 bushels of corn was made where Accent was used versus 113 bushels for the untreated plot. Option gave slightly less control, he says.

The potential for crop injury from Accent increases significantly at the V6 to V7 stage, says Lee. The potential for injury from Prowl occurs earlier in the growing season.

“There are some rotation restrictions with Accent, but that's usually not a problem in Georgia with our 10-month rotations in cotton a peanuts. Soybeans require a shorter period of time.”

Turning to problem weeds in Georgia, Lee says nutsedge doesn't offer much competition for corn. Studies show only about a 2-percent loss when infestations reach 800 plants per 100 row feet. Eradicane, Dual, glyphosate, Basagran and Permit all can be effective against nutsedge, he says.

As for pigweed, several pre-emergence and postemergence materials can be effective, he says, including atrazine, Dual and Outlook. Timing is critical for effective, efficient control, he adds.